This invention relates broadly to an improvement in nuclear fuel elements for use in the core of nuclear fission reactors, and more particularly to an improved nuclear fuel element having therein a getter comprising a bimetallic composite having the components of (1) a metallic substrate having thereon (2) a coating capable of being fractured and covering at least a portion of the substrate. The coating is capable of gettering a source of hydrogen such as hydrogen, water vapor and hydrogen-containing compounds and gases.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which may have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves two primary purposes: first, to prevent contact and chemical reactions between the nuclear fuel and either the coolant or moderator if present, or both if both the coolant and moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or moderator or both if both the coolant, and moderator are present. Common cladding materials are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium (columbium), certain magnesium alloys, and others. The failure of the cladding, due to the build-up of gas pressure or other reasons, can contaminate the coolant or moderator and the associated systems with radioactive longlived products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to the reactivity of these materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent materials as a nuclear fuel cladding since they have low neutron absorption cross sections and at temperatures below about 600.degree. F are extremely stable and non-reactive in the presence of demineralized water or stream which are commonly used as reactor coolants and moderators. Within the confines of a sealed fuel element, however, the hydrogen gas generated by the slow reaction between the cladding and residual water may build up to levels which under certain conditions can result in localized hydriding of the cladding with concurrent deterioration in the mechanical properties of the cladding. The cladding is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide over a wide range of temperatures.
The zirconium cladding of a nuclear fuel element is exposed to one or more of the gases given above during irradiation in a nuclear reactor in spite of the fact that these gases may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from the ambient atmosphere during manufacture of the cladding and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and others used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture and especially during irradiation. Particulate refractory and ceramic compositions, such as uranium dioxide powder and other powders used as nuclear fuel, have been known to release even larger quantities of the aforementioned gases during irradiation. These gases react with zirconium cladding containing the nuclear fuel. This reaction can result in the embrittlement of the cladding which endangers the integrity of the fuel element. Although water and water vapor may not react directly to produce this result, at high temperatures water vapor does react with zirconium and zirconium alloys to produce hydrogen and this gas further reacts locally with the zirconium and zirconium alloys to cause embrittlement. These undesirable results are exaggerated by the release of these residual gases within the sealed metal-clad fuel element since it increases the internal pressure within the element and thus introduces stresses in the presence of corrosive conditions not anticipated in the original design of the cladding.
In light of the foregoing, it has been found desirable to minimize water, water vapor and other gases, especially hydrogen, reactive with the cladding from inside the fuel element throughout the time the fuel element is used in the operation of nuclear power plants. One such approach has been to find materials which will chemically react rapidly with the water, water vapor and other gases to eliminate these from the interior of the cladding, and such materials are called getters. While several getters for water and water vapor have been found, such as the zirconium-titanium getter set forth in U.S. Pat. Nos. 2,926,981 and 3,141,830, it has remained desirable to develop a getter having equal or even greater rapidity of reaction with hydrogen and hydrogen containing compounds, and having the feature of producing negligible hydrogen gas during the reaction with moisture.
It is also desirable to eliminate one problem associated with the use of zirconium and zirconium alloys as a getter in nuclear fuel, and this problem is the tendency of zirconium and zirconium alloys to form a protective continuous film of zirconium oxide. This film inhibits the reaction of the underlying zirconium with the material to be gettered. The zirconium oxide film forms on all surfaces of a zirconium material exposed to an atmosphere containing oxygen at reactor operating temperatures.